The present invention pertains to the treatment of ferromagnetic material to refine magnetic domain spacing and the resultant products produced thereby. It is especially concerned with the non-physical contact scribing of ferromagnetic sheet and laminations, and the products produced thereby.
The development of high permeability grain oriented silicon steel for use in magnetic cores, (e.g. transformer cores) resulted in a significant reduction in core loss, especially at inductions greater than 1.5 T (15 kG). This reduction in loss has been achieved primarily by improvements in the degree of grain orientation. Separation of the components contributing to the overall core loss has shown that the improved losses obtained are due to a reduction in the hysteresis component of the core loss. Further loss reductions can be achieved by refining the 180.degree. domain wall spacing, which results in a lowering of the eddy current component of core loss.
Over the past several years techniques have been developed to reduce the domain wall spacing by changing the magnetostatic or the magnetoelastic energy in the sheet. Insulative coatings that apply a tensile stress parallel to the rolling direction have been effective in reducing the domain wall spacing and the core loss. Mechanical, or physical, scribing transverse to the sheet rolling direction is another technique that has been found to be effective in reducing domain spacing and lowering the losses. The disadvantages of mechanical scribing are that the insulative coating is disturbed, and the space factor is decreased.
Efforts to obtain the advantages of scribing without the aforementioned disadvantages have centered around the use of pulsed laser scribing techniques. It is known that irradiation of an iron-silicon alloy by a laser pulse of sufficient power density can vaporize material at the alloy surface or insulative coating surface causing a pressure shock wave to travel through the alloy causing dislocations and twins (see A. H. Clauer et al, "Pulsed Laser Induced Deformation in an Fe-3Wt Pct Si Alloy," Metallurgical Transactions A, Vol. 8A, January 1977 pp. 119-125). This deformation, like the deformation produced by mechanical scribing, can be used to control domain spacing. In fact, pulsed lasers have been applied to grain oriented electromagnetic steel sheet to produce shock wave induced arrays of deformation. (see, for example, U.S. Pat. No. 4,293,350 and French Patent Application No. 80/22231 published on Apr. 30, 1981 Publication No. 2,468,191). It has however been reported that pulsed laser scribing after an insulating film has been applied to the major surfaces of the ferromagnetic steel sheet is likely to result in removal of the insulating film in the irradiated areas causing a deterioration in the film's insulating properties, corrosion protection properties and ability to withstand high voltage (see for example European Patent Application Publication No. 0033878 A2). While this coating damage can be repaired by recoating after laser scribing, the coating applied should be curable at a temperature below about 600.degree. C. to avoid annealing out the beneficial effects of laser scribing. Recoating is also undesirable because of it being an additional step in the manufacturing process.
In accordance with the present invention applicants have discovered that the domain size, and therefore the watt losses, in a ferromagnetic sheet material can be reduced by a process involving the rapid heating of narrow bands of the ferromagnetic sheet material to an elevated temperature, preferably below the material's solidus, and immediately thereafter self-quenching the heated material. In this manner it is believed that plastic deformation is produced within the thermally treated material due to the stresses developed in it because of the constraints imposed on its thermal expansion by the surrounding relatively cold material.
It has also been surprisingly discovered that when the method of scribing according to this invention is applied to ferromagnetic material which has been previously coated with a film of electrically insulative material that the ferromagnetic material can be scribed, while maintaining the insulative properties of the film. The method according to this invention preferably does not change the surface roughness of the film or cause the film to melt.
Also in accordance with the present invention it is preferred that the thermal treatment used to produce the scribe lines be conducted by an energy beam operating in a continuous mode as it impinges on and travels across the sheet. It has been found that a CW (continuous wave) laser beam is useful for these purposes.
Neodymium YAG or Neodymium glass and CO.sub.2 lasers are suitable for use in the present invention.
The material to be treated by this process includes both coated and uncoated ferromagnetic sheet material having a large domain size, such as that found in grain oriented and high permeability grain oriented silicon electrical steels. This invention may also be applied to iron-nickel alloys, iron-cobalt alloys, iron-nickel-cobalt alloys and amorphous ferromagnetic materials, which can also benefit by the reduction in domain size produced by scribing in accordance with the present invention.
The aforementioned and other aspects of the present invention will become more apparent upon examination of the drawings, which are briefly described below, in conjunction with the detailed description of the invention.